Enforce dangling commas

[jennasalau]

I'm kind of expecting a healthy debate over this one.

But I would like to propose we enforce dangling comma's in our lint/style guide for multiline statements.

https://medium.com/@nikgraf/why-you-should-enforce-dangling-commas-for-multiline-statements-d034c98e36f8

What do you all think?

[dkeeghan]

Previously I would have said no, but considering that we're now not supporting IE8 and given the benefits mentioned in the article I'm sold.

[lachlanmcdonald]

For arrays\object definitions, I have no complaints. Not a fan of this though:
http://babeljs.io/docs/plugins/syntax-trailing-function-commas/

[dkeeghan]

@lachlanmcdonald agreed, functions are different

[jennasalau]

I'm glad that at least you're all positive on 80% of it.

@lachlanmcdonald dangling comma's will feel weird until you get used to them. Hopefully at that point I can win you over on function parameters too in a later PR :).. They just so happen to be in proposal for ES7 too https://github.com/tc39/proposal-trailing-function-commas

Just to be clear its only when you split them over multiple lines. The rule is not enforced if everything is on the same line.

I'll update the PR tomorrow to remove the rule from functions for now.

[dkeeghan]

If it's not on single line functions I'd say leave it in then for multiline for the sake of consistency

[keeganstreet]

I like it for the benefit of cleaner diffs. I would go all out and enforce it on all multi-line statements, including function arguments, for consistency. I think multi-line arguments are quite rare anyway. If a function has that many arguments it probably needs to be simplified.